Explain the manufacture of Potassium dichromate from chromite ore.

Explanation

Potassium dichromate `K_(2)Cr_(2)O_(7)` is obtained from chromite ore `(FeCr_(2)O_(4))` in following steps:
Step 1: Chromite ore is fused with molten `Na_(2)CO_(3)` in the presence of air, `Na_(2)CrO_(4)` and `Fe_(2)O_(3)` are formed. `Fe_(2)O_(3)` is water insoluble while `Na_(2)CrO_(4)` is water soluble hence separation is done.
`4FeCr_(2)O_(3)+8Na_(2)CO_(3)+7O_(2)rarr 8Na_(2)CrO_(4)+Fe_(2)O_(3)+8CO_(2)`
Step 2: Aqueous solution of `Na_(2)CO_(3)` is acidified with concentrated `H_(2)SO_(4)` to form `Na_(2)Cr_(2)O_(7)`
`2Na_(2)CrO_(7)+H_(2)SO_(4)rarr Na_(2)Cr_(2)O_(7)+Na_(2)SO_(4)+H_(2)O`
Step 3: Sodium dichromate `(Na_(2)Cr_(2)O_(7))` is more soluble and less stable than `K_(2)Cr_(2)O_(7)` hence `Na_(2)Cr_(2)O_(7)` changes to `K_(2)Cr_(2)O_(7)` on reaction with `KCI`. On cooling concentrated solution, orange crystals of `K_(2)Cr_(2)O_(7)` are obtained.
`Na_(2)Cr_(2)O_(7)+2KCl rarr K_(2)Cr_(2)O_(7)+2NaCl`